Pahiatua residents will have to boil water for at least three days due to bacterial contamination.
The Tararua District Council issued a boil water notice after test results showed the presence of E. coli in the township's drinking water.
The community's water is also being chlorinated.
Council workers are going door to door around homes and businesses, and all schools, rest homes, medical centres, sports centres and day-centres have been phoned.
Tararua Mayor Roly Ellis said they would not know the extent of the contamination until tomorrow.